Hydrothermal activity, especially high-temperature discharge, is commonly thought to require the presence of an axial magma
chamber (AMC), but this association has not been examined systematically. The availability of six lengthy (170–560 km) ridge
sections with continuous surveys of both AMCs and hydrothermal plumes now makes it possible to quantitatively compare the
distribution and intensity of hydrothermal activity to AMC extent, depth, and, at a few locations, melt content. These six
ridge sections span spreading rates from 55 to 145 mm/a and total 20 second-order segments. At the section (multisegment)
scale the linear incidence of hydrothermal plumes increases with AMC incidence (AMCI, r2 = 0.64), excluding the hot spot–affected Galápagos spreading center. For all six sections, plume incidence increases as AMC
depth below the seafloor decreases (AMCZ, r2 = 0.66). At the second-order segment scale, plume incidence is poorly correlated with both AMCI (r2 = 0.12) and AMCZ (r2 = 0.25). Finally, at the subsegment, or local, scale (0.75-km-long bins), plume intensity increases as AMCZ shallows (r2 = 0.85). Of bins where plumes are most intense and thus closest to their seafloor sources, 68 ± 13% lie directly over an
AMC, as do at least 37 of the 40 known high-temperature vent fields. The data also allow tests of other hypotheses linking
AMC properties and hydrothermal activity. Existing data, though still sparse, do not support the hypothesis that lenses of
melt-rich magma preferentially support vigorous, long-lasting venting. Also, the suggestion that increased hydrothermal cooling
within a segment locally depresses AMCZ finds no support within any ridge section. Evidence for magma bodies is much scarcer on slow spreading ridges, but the data
are nevertheless consistent with those from faster ridges. Observations from all spreading rates thus demonstrate that high-temperature
vent fields are almost universally associated with the presence or inference of magma; “hot rock” or other nonmagmatic heat
sources are insufficient.